1. Field of the Invention
The present invention relates to a high density magnetic recording medium. More particularly, the present invention relates to a magnetic recording medium having a thin magnetic film.
2. Description of the Related Art
In recent years, magnetic recording media have been digitized in order to avoid signal deterioration by many times of copies. The magnetic recording media have also been needed to have a high density, since they have to record a large quantity of data. Fine metal magnetic powders having high saturated magnetic flux density and hexagonal tabular-shaped barium ferrite magnetic powders have been practically used.
On the other hand, for providing a high density, it is necessary to consider thickness loss and self-demagnetization of the medium. In view of the above, the magnetic layer is to be thin. However, if the magnetic layer is thinned, surface properties of a substrate may unfavorably affect a surface of the magnetic layer, which may deteriorate electro-magnetic conversion properties. It has been conventionally proposed that a non-magnetic layer comprising a thermosetting resin is disposed on the surface of the substrate, and a magnetic layer is disposed on the substrate via the non-magnetic layer. Hitherto, in order to improve the durability of the non-magnetic layer, a number of thermosetting resin binders have been proposed. Recently, some radiation curing type binders are proposed and used to provide a stronger coating film.
Japanese Examined Patent Application Publication Nos. 6-52566 and 1-30221 describe that a radiation curing type binder is used for a non-magnetic coating agent. However, it is difficult to produce a coating agent for a non-magnetic layer having good dispersibility using such a radiation curing type binder disclosed in the publications in order to increase a coating strength. As a result, electromagnetic conversion properties of the magnetic recording medium tend to be deteriorated.
Japanese Unexamined Patent Application Publication Nos. 2001-84569 and 2001-84570 both filed by the present applicant describe that a polyurethane resin having a polar group is used as a radiation curing type binder for a non-magnetic under layer. Specifically, the former publication discloses that the nonmagnetic layer contains as a pigment at least acid carbon black that has a pH of less than 6, and also contains the radiation curing type binder including a polyur thane resin having at least a basic polar group. The latter publication discloses that the nonmagnetic layer contains as a pigment at least carbon black, and also contains the radiation curing type binder including a polyurethane resin having at least a basic polar group.
In the prior art proposed by the present applicant, the non-magnetic layer containing carbon black has improved strength, electromagnetic conversion properties and durability. However, a magnetic recording medium with a higher density and a thinner magnetic layer having a thickness of less than 0.30 μm is required at present. Accordingly, the prior art non-magnetic layer has insufficient dispersibility in the case of such a magnetic recording medium.